1. Field of the Invention
The present invention relates to a switching voltage regulator, and more particularly, to a switching voltage regulator with circuit improving efficiency under light load condition.
2. Description of the Related Art
A voltage regulator serves the function of transforming an input voltage to a regulated output voltage. A voltage regulator typically can be categorized as a linear voltage regulator or a switching voltage regulator. A linear voltage regulator utilizes passive components, such as variable resistors, to provide a continuous current from the input terminal to the output terminal. A switching voltage regulator, on the other hand, utilizes a pair of switches connected in series, activated interchangeably, to provide a current to the output terminal.
Because the power efficiency of the linear voltage regulator is generally below 65%, this architecture is not suitable for the hand-held devices, mainly powered from battery. Therefore, the power supply apparatus in mobile electronic devices such as notebook computers or cellular phones is mainly implemented by switching voltage regulators.
FIG. 1 shows a conventional switching voltage regulator. The switching voltage regulator 10 comprises a switching circuit 20, an output circuit 30, a feedback circuit 35, a pulse width modulation (PWM) control circuit 40 and a compensation circuit 47. The switching circuit 20 comprises a PMOS high side transistor 21 and a low side NMOS transistor 22. The source electrode of the high side transistor 21 is connected to an input voltage. The source electrode of the low side transistor 22 is grounded. The drain electrode of the low side transistor 22 is connected to the drain electrode of the high side transistor 21. The output circuit 30 comprises an inductor L1, a capacitor C1 and a parasitical resistor R1. The inductor L1, the capacitor C1 and the parasitical resistor R1 form a low pass filter, transforming the output current IL of the switching circuit 20 into a regulated voltage across a load circuit RL. The feedback circuit 35 comprises resistors R2 and R3. The PWM control circuit 40 comprises an error amplifier 41, a PWM comparator 42, a pulse generator 43, a high side driver 44, a low side driver 45 and a current-to-voltage amplifier 46. The resistor R1 is connected to the output terminal of the switching voltage regulator 10. The resistor R2 connects the resistor R1 to ground. The input terminals of the error amplifier 41 are connected to a reference voltage and the common node of the resistors R2 and R3, respectively. The negative input terminal of the PWM comparator 42 is connected to the output terminal of the error amplifier 41 and the compensation circuit 47. The positive input terminal of the PWM comparator 42 receives the sum of the output terminal of the current-to-voltage amplifier 46 and a slope compensation signal. The input terminals of the pulse generator 43 are connected to the output terminal of the PWM comparator 42 and a clock signal. The input terminals of the high side driver 44 and the low side driver 45 are both connected to the output terminal of the pulse generator 43. The output terminals of the high side driver 44 and the low side driver 45 are connected to the gate electrodes of the high side transistor 21 and the low side transistor 22, respectively. The current-to-voltage amplifier 46 transforms the current flowing through the inductor L1 into a voltage signal.
The error amplifier 41 regulates the output voltage of the switching voltage regulator 10 by connecting the reference voltage to its non-inverting terminal and the divided voltage of the output voltage of the switching voltage regulator 10 to its inverting terminal.
When in a normal mode, the output voltage of the pulse generator 43 is low at the beginning of each cycle of the clock signal. The high side driver 44 activates the high side transistor 21. The low side driver 45 deactivates the low side transistor 22. The output voltage VL of the switching circuit 20 equals to the input voltage, while the output current IL of the switching circuit 20 is increasing. When the output current IL exceeds a threshold such that the sum of the transformed voltage of the current-to-voltage amplifier 46 and the slope compensation signal is greater than the output voltage of the error amplifier 41, the pulse generator 43 outputs a pulse signal of a fixed duration. The high side driver 44 deactivates the high side transistor 21. The low side driver 45 activates the low side transistor 22. The output current IL starts to drop. When the next cycle of the clock signal resets the output voltage of the pulse generator to low, the high side transistor 21 is reactivated, and the low side transistor 22 is deactivated.
FIG. 2 shows the waveforms of the output voltage VL and the output current IL of the switching voltage regulator 10. As shown in FIG. 2, the waveform of the output voltage VL is a square wave, and the waveform of the output current IL is a triangular wave.
When the load circuit RL becomes large, i.e., when the DC level of the output current IL of the switching voltage regulator 10 is low, the power efficiency of the switching voltage regulator 10 declines due to the switch loss. Moreover, when the output current IL of the switching voltage regulator 10 drops below zero ampere, the switching voltage regulator 10 drains power from the load circuit RL. Therefore, there is a need to design a switching voltage regulator that can maintain high power efficiency as it operates under the light load condition.